Introduction
When filtering certain types of materials a dense deposit can form on the filter bed and cause the filter to become clogged very quickly. This requires that the filter be cleaned frequently. The life of the filter bed can be extended by mixing a filter aid (commonly referred to as "bodyfeed") such as diatomaceous earth with the liquid being filtered and/or as a pre-coat for the filter media. The enhanced efficiency of a "bodyfeed" filter system of this type more than pays for the added cost of the system and the filter aid.
In many cases the load on the filter is not constant over time. In these cases, the filter aid should be added at a rate consistent with the level of contamination of the liquid being filtered. When this approach is taken, the operating time of the time of the filter can be increased even more.
The present method of bodyfeeding a pressure or vacuum filter requires operator input to control the rate of bodyfeed injection. The most common procedure of optimizing this injection rate is to adjust the "bodyfeed injection rate time" to a rate high enough to accommodate the maximum foreseeable dirt load. This results in a shorter cycle time that would be otherwise witnessed if the injection rate was adjusted to the load at any time. Due to the tedious as well as time consuming task this method would take, it is not the normal practice in an industrial setting. Some examples of inefficient timer settings would be the process equipment that generates the dirt load being at idle for some period of time; if the timer were set for the anticipation of a given dirt load and there was none generated, this would result in an inappreciable pressure increase and a depletion of the bodyfeed tank. When the dirt load resumed, the filter would immediately go to pressure. Another example would be if a low injection rate was set in anticipation of a light dirt load; if the load increased a short filter cycle would result.
A previous solution to this problem was the use of an electro-mechanical device that changed the bodyfeed rate of a pressure diatomaceous earth water filter. This system was used to filter suspended solids from river water. Due to variables in solids concentrations as a result of seasonal changes as well as vessel traffic, this device was needed. The sensing device was a turbidimeter which measured the loss of light across a fixed gap of incoming water. This signal was then used to select from a group of preset times that would vary the bodyfeed injection rate. The higher the turbidity, the higher the injection rate. Due to the inherent problems associated with turbidimeters used in industrial applications such as the fouling of the sensing cell and the resulting need of maintenance and recalibration, this system proved less than desirable.
This system did not feature the integration of a "bodyfeed level signal" which supplies another signal to better optimize the length of filter cycle run. As the dirt load increased the bodyfeed rate increased the resulting pressure would increase closer to the termination pressure. This would sometimes result in the bodyfeed tank being less than empty. Since the bodyfeed tank has the desired amount of filter aid needed to form the optimum cake thickness, any filter termination occurring with less than an empty bodyfeed tank would result in an inefficient use of the disposable filter aid support septum.